This title may be click bait, but...

[@rt]

What's the difference between 7403 & 7401?
Answer: 2

But really, the only difference I see is the pinout.
Was it really necessary to design two chips in 7400 series for that?
Cheers, Brek.

[T3sl4co1l]

Open collector outputs.

Tim

[Benta]

Both are open collector. No difference except pinout.

[Neomys Sapiens]

What is interesting is that the 54/7401 had different layouts trough the various series and packages, while for the '03 they seem to be all similar.
(from TI The_TTL_Data_Book_For_Design_Engineers_2ed_1981)

[@rt]

I guess I was lucky I used, and already wired HD74LS01, and it was as expected (top left).

What is interesting is that the 54/7401 had different layouts trough the various series and packages, while for the '03 they seem to be all similar.
(from TI The_TTL_Data_Book_For_Design_Engineers_2ed_1981)

[SeanB]

W and T packages are the ceramic flatpack versions, which were also available in many of the original TTL family as an option. As well the 03 has high voltage capable outputs, which can withstand up to 30V on the open collector, used for interfacing to things like reed relays at 24V and such things, and also used as interface to NMOS logic as well.

[Benta]

As well the 03 has high voltage capable outputs, which can withstand up to 30V on the open collector

Both the 7401 and the 7403 are rated at 7 V maximum output off voltage. Source: www.ti.com datasheets.

[SeanB]

Both the 7401 and the 7403 are rated at 7 V maximum output off voltage. Source: www.ti.com datasheets.

Sorry, my bad, went to look at the paper copy of the TI datasheet, and yes, they are 7V rated, I was thinking of the 06 and 07, which were 30V rated for the outputs.

Funny thing is the rest of the chip is still only rated for 7V, though you will find that many will work at 10V with nothing other than running pretty hot. Got a wall ornament board where that hapenned, the power supply decided to apply unregulated 14V to the TTL, and the only IC to fail was a SFC5406PM3 30V capable open collector gate, which was easy to find, as it had unsoldered the can of the flatpack lid.

[Neomys Sapiens]

Funny thing is the rest of the chip is still only rated for 7V, though you will find that many will work at 10V with nothing other than running pretty hot. Got a wall ornament board where that hapenned, the power supply decided to apply unregulated 14V to the TTL, and the only IC to fail was a SFC5406PM3 30V capable open collector gate, which was easy to find, as it had unsoldered the can of the flatpack lid.

How high did you fly that thing?